As an emission type electronic displaying device, there is an electroluminescent display (hereinafter referred to as ELD). As devices constituting the ELD, there are mentioned an inorganic electroluminescent element and an organic electroluminescent element (hereinafter referred to as organic EL element).
The inorganic electroluminescent element has been used for a plane-shaped light source, but a high voltage alternating current has been required to drive the element.
An organic EL element has a structure in which a light emission layer containing a light emission compound is arranged between a cathode and an anode, and an electron and a hole are injected into the light emission layer and recombined to form an exciton. The element emits light, utilizing light (fluorescent light or phosphorescent light) generated by inactivation of the exciton, and the element can emit light by applying a relatively low voltage of from several volts to several decade volts. The element has a wide viewing angle and high visualization since the element is of self light emission type. Further, the element is a thin, complete solid device, and therefore, the element is noted from the viewpoint of space saving and portability.
However, development of an organic EL element for practical use is demanded which efficiently emits light with high luminance at a lower power.
High emission luminance and long lifetime is attained in Japanese Patent No. 3093796 by doping a slight amount of a phosphor in stilbene derivatives, distyrylarylene derivatives or tristyrylarylene derivatives.
An element is known which comprises an organic light emission layer containing an 8-hydroxyquinoline aluminum complex as a host compound doped with a slight amount of a phosphor in Japanese Patent O.P.I. Publication No. 63-264692, and an element is known which comprises an organic light emission layer containing an 8-hydroxyquinoline aluminum complex as a host compound doped with a quinacridone type dye in Japanese Patent O.P.I. Publication No. 3-255190.
When light emitted through excited singlet state is used as in the above, the upper limit of externally taking-out quantum efficiency (η) is considered to be at most 5%, as the generation ratio of singlet excited species to triplet excited species is 1:3, that is, the generation probability of excited species capable of emitting light is 25%, and further, external light emission efficiency is 20%.
Since an organic EL element, employing phosphorescence through the excitation triplet, was reported by Prinston University (see M. A. Baldo et al, Nature, 395, p. 151-154 (1998)), study on materials emitting phosphorescence at room temperature has been actively made.
For example, such an organic EL element is disclosed in M. A. Baldo et al., Nature, 403, 17, p. 750-753 (2000) or U.S. Pat. No. 6,097,147.
As the upper limit of the internal quantum efficiency of the excitation triplet is 100%, the light emission efficiency of the excitation triplet is theoretically four times that of the excited singlet. Such an organic EL element has possibility that exhibits the same performance as a cold cathode tube, and its application to illumination is watched.
Many compounds, mainly heavy metal complexes such as iridium complexes are synthesized and studied in for example, S. Lamansky et al, J. Am. Chem. Soc., 123, 4304 (2001).
An example employing tris(2-phenylpyridine)iridium as a dopant is studied in M. A Baldo et al, Nature, 403, 17, p. 750-753 (2000) above.
Further, M. E. Tompson et at studies an example employing as a dopant L2Ir(acae) such as (ppy)2Ir (acac) in The 10th International Workshop on Inorganic and Organic Electroluminescence (EL ' 00, Hamamatsu), and Moon-Jae You n. Og, Tetsuo Tsutsui et at an example employing as a dopant tris(2-p-tolylpyridine)iridium {Ir(ptpy)3} or tris(benzo-N-quinoline)iridium {Ir(bzq)3} in The 10th International Workshop on Inorganic and Organic Electroluminescence (EL ' 00, Hamamatsu). In addition, these metal complexes are generally called orthometalated iridium complexes.
As described above, attempt for preparing an element employing various iridium complexes is made in S. Lamansky et al, J. Am. Chem. Soc., 123, 4304 (2001) or in Japanese Patent O.P.I. Publication No. 2001-247859.
Further, to obtain high emission efficiency, Ikai et al utilized a hole transporting compound as a host of a phosphorescent compound at The 10th International Workshops on Inorganic and Organic Electroluminescence (EL'00, Hamamatsu). Further, M. E. Tompson et al. utilized various types of electron transporting materials doped with a new iridium complex as a host of a phosphorescent compound.
An organic EL element fitted with the iridium complex is prepared generally via evaporation. Studies of an organic EL element prepared by a coating method have been actively done, but it is presently difficult to prepare the organic EL element via coating since the iridium complex exhibits low solubility. Thus, it is desired to improve solubility of the iridium complex.
Further, orthometalated complexes in which iridium as a center metal is replaced by platinum are also watched. Regarding these complexes, there are known many kinds of complexes having characteristics in the ligands, which are disclosed in Japanese Patent O.P.I. Publication Nos. 2002-332291, 2002-332292, 2002-338588, 2002-226495, and 2002-234894, for example.
Light emission elements employing the above compounds exhibit greatly unproved emission luminance and emission efficiency as compared to conventional elements, because the light emission arises from phosphorescence, but they have a problem in that emission lifetime is low as compared to conventional elements. In this way, in the case of a light emitting material exhibiting high phosphorescence efficiency, it is difficult to improve realization of shorter light emission wavelength as well as emission lifetime, whereby presently, practically sufficient tolerable performance has not yet been achieved.
As a material to improve the performance, for example, known is an Ir complex or a Pt complex each having a phenyl imidazole derivative as a ligand in WO 02/15645 and WO 05/7767. However, light emission efficiency and lifetime of the element of each of these complexes are not sufficiently satisfactory, and further improvement of the light emission efficiency and the lifetime are desired to be improved.
A vacuum evaporation method is conventionally utilized as a method of manufacturing an organic EL element, but since in the case of the conventional vacuum evaporation method, high vacuum is required forte operation, the manufacturable member is limited in size, and a step of taking a member in and out is also required at the same time. Thus, the conventional vacuum evaporation method is rejected as unsuitable for the continuous production.
On the other hand, as a continuously manufacturable means, a method of employing an EL material solution is disclosed (refer to Patent Document 1, for example), and a low-molecular material and a polymeric material are to be usable as the EL material.
However, in the case of coating with a low-molecular material, a lower layer and an upper layer are difficult to be incorporated during formation of a multilayer, resulting in difficulty of obtaining an EL element exhibiting high performance.
Further in the case of a polymeric material, there appears a problem such that a refining method available with a low-molecular material such as a recrystallization method, a sublimation refining method, and a silica column refining method cannot be utilized, and impurities contained in a monomer as raw material to prepare a polymer are difficult to be removed. Thus, this reason leads to a factor to degrade light emission lifetime.    Patent Document 1: Japanese Patent O.P.I. Publication No. 2001-297882